This invention relates to improved well instruments, and particularly to survey devices which are to be lowered into a drill string or other string of well pipe for the purpose of recording directional information.
A well known type of survey instrument utilized in drilling operations is the `single shot` survey tool which normally includes a magnetic compass and a camera for photographing the compass at a location deep within a well. The compass assembly may include an inclinometer, so that the photographic record can indicate both the inclination of the portion of the drill string within which the picture is taken and the direction of that inclination. This information may then be utilized for determining the direction in which further drilling should be aimed.
In order to enable response of the compass to the earth's magnetic field, the drill string may be formed to include a short section of non-magnetic pipe at the survey location within which the instrument is received while the picture is taken. For use with a drill string having such a non-magnetic section, U.S. Pat. No. 4,365,197 issued Dec. 21, 1982 to Pyatt et al. on "Identification Of Pipe Material In Wells" has disclosed a sensor unit adapted to be lowered into the drill string with the instrument, and which includes a detector constructed to respond to arrival of the instrument within the non-magnetic section of pipe and upon such arrival automatically initiate a cycle of operation of the camera. This eliminates the necessity for attempting to estimate the time that the lowering operation will take, and then presetting the instrument to take a picture at that time, a procedure which almost invariably results in a substantial waste of rig time.
One problem which has been encountered in the use of automatic sensors of the above discussed type is that of reliably ascertaining before lowering of the device into a well that the device is in good working condition and will in fact function to make the desired survey record. If the circuitry of the automatic control system is inoperative for any reason, the entire period of time required for lowering the instrument and then retreiving it may be lost, with no survey information being obtained. Further, even if the unit is checked at the surface of the earth and the camera lamps operate properly at that location, this does not assure that the battery voltage level after the test procedure will remain high enough to assure proper illumination of the lamps again in the well.